Make rebar3_archive_plugin conditional on rebar3 (#112)

* Make it work with rebar2:

Make the rebar3_archive_code plugin included only for rebar3.

* Include the right path to the tests:

It looks like ./rebar3 eunit is broken in the master branch.
Somehow it does not compile the tests. Had to edit the
rebar.config and provide a path to the unit tests.

Author: NikolaBorisov

rebar.config

@@ -43,7 +43,9 @@
                       {top_level_readme,
                        {"./README.md",
                         "https://github.com/deadtrickster/prometheus.erl"}}]}]},
-            {test, [{plugins, [{coveralls, "1.4.0"},
+            {test, [
+                    {eunit_compile_opts, [{src_dirs, ["src", "test/eunit"]}]},
+                    {plugins, [{coveralls, "1.4.0"},
                                {rebar3_elvis_plugin, "0.0.4"}]}]},
             {gpb, [{plugins, [{rebar3_gpb_plugin, "2.7.0"}]},
                    {provider_hooks, [{pre,  [{compile, {protobuf, compile}}]},
@@ -56,4 +58,3 @@
                          {deps, [{'erlang-color',
                                   {git, "https://github.com/julianduque/erlang-color", {branch, "master"}}}]}]}]}.
 
-{plugins, [{rebar3_archive_plugin, "0.0.1"}]}.

rebar.config.script

@@ -8,7 +8,7 @@ end,
 
 case erlang:function_exported(rebar3, main, 1) of
     true -> % rebar3
-        TravisConfig;
+        lists:keystore(plugins, 1, TravisConfig, {plugins, [{rebar3_archive_plugin, "0.0.1"}]});
     false -> % rebar 2.x or older
         lists:keystore(proto_opts, 1, TravisConfig, {proto_opts, [{src_dirs, []}]})
 end.